This invention relates to expandable thermoplastic polymer beads and method of producing same, and more particularly to such expandable beads which substantially does not lose foaming agent absorbed therein and which substantially improved elasticity, expandability, strength and moldability.
In the prior art, there are a number of different kinds of expandable polymer beads, for example, use of a styrene based monomer added to a polyethylene-based resin in an aqueous medium, polymerizing the two together and then impregnating a foaming agent therein. These modified beads show a certain amount of improved elasticity in their molded expanded products as compared to the beads made of polystyrene alone, and show a relatively high degree of expansion as compared to beads made of polyolefines alone. Also, such modified beads readily absorb foaming agents and produce relatively good expanded pre-foams. However, disadvantageously, such prior art modified beads tend to lose their foaming agents a short time after impregnation. Thus, these modified beads, although having the properties of good degree of expansion and good elasticity, cannot be stored for any substantial length of time and thus must be used almost immediately by the user.